To realize accuracy and stability of optical modulation, it is important to stabilize the luminous intensity of the traveling wave generated. However, it is known that since in general, an acousto-optic medium significantly differs in acoustic impedance from air, ultrasonic waves reflect on the boundary surface of the acousto-optic medium to degrade the stability of ultrasonic wave travelling in crystal. There is a conventional technique developed to avoid the degradation. In this technique, the surface of the acousto-optic medium opposed to a surface of a piezoelectric transducer and the surface of the piezoelectric transducer are not parallel thereto to prevent direct reflection, and an ultrasonic wave absorbing material is employed to absorb ultrasonic waves.
However, such an absorbing material does not exist that has an acoustic impedance completely equal to that of the acousto-optic medium, can achieve sufficient ultrasonic wave attenuation, and can be easily installed and produced at low cost. Accordingly, in the conventional acousto-optic modulators, certain ultrasonic reflection waves exist, which may cause various types of degradation of modulation accuracy.